In a furnace, efficiency is increased when the heat from the furnace is passed in heat-exchange relationship with the batch material being supplied to the melting furnace. The batch can thus be preheated to elevated temperatures to save significant amounts of energy subsequently required to melt the batch.
Preferably, the heat-softenable batch material is in the form of balls or pellets in the heat-exchange chamber through which the hot gases are passed. However, it has been discovered that the pellet size must be substantially uniform. Otherwise, pellets of varying sizes tend to nest and provide excessive restriction of the flow of the gases past the pellets in the chamber. It has also been discovered that pellet size is important in addition to uniformity. If the pellets are too small, again undue restriction to the flow of the hot gases results. If the pellets are too large, their surface-to-weight ratio is accordingly reduced and the heat transferred to them is accordingly decreased. Also, trapped moisture in the larger pellets may turn to steam and cause the pellets to explode. Specifically, it has been found that pellets of one-half inch nominal diameter with a range from three-eighths inch to five-eighths inch in diameter are the ultimate for obtaining maximum heat transfer from the hot exhaust gases to the pellets.
The pellets of the heat-softenable batch material preferably are made in a modified commercially-available pelletizer. The components of the batch are mixed together and then supplied to the pelletizer. During transportation to the pelletizer, the batch components tend to segregate so that the actual batch supplied to the pelletizer will vary, even though the final pellets produced and supplied to the melting furnace or unit average out so that the short variations are not material. However, the short variations in the batch components tend to affect the pellet-forming ability of the batch and the size of the pellets produced, other factors being constant. The feed rate of the batch to the pelletizer will also vary and thereby affect pellet forming and pellet size. Liquid, and specifically water, is also supplied to the pelletizer near the batch supply. With the batch component or quantity variation, different size pellets will result when the water quantity is held constant. However, it has been found that the water quantity, or the ratio of the batch to the water, will also affect the pellet size, with more water resulting in larger pellets and less water resulting in smaller pellets, at least in most instances.
In the prior art, considerable difficulty has been experienced with pelletizing. Specifically difficult to develop was an acceptable manner of controlling the size of the pellets produced.
The difficulty has resulted in an uneconomical operation of a pelletizer whereas an excessively large portion of the pelletizer output falls above or below acceptable size limits and must be reground for later use in the pelletizer.
One device for controlling the size of pellets produced from a rotary pelletizer is disclosed in co-pending application Ser. No. 932,244, filed Aug. 9, 1978, and assigned to the common assignee. That application discloses a system for evaluating the size of completely formed pellets by separating the pellets into sized groups and weighing each separate group.
Other devices for controlling pellet size produced in a rotary pelletizer are disclosed in copending application Ser. No. 049,865, filed June 18, 1979, which is a continuation of Ser. No. 809,595, filed June 24, 1977, now abandoned and copending application Ser. No. 095,268, filed Nov. 29, 1979, which is a continuation-in-part of Ser. No. 974,470, filed Dec. 29, 1978, now abandoned, and both applications are assigned to the common assignee, and U.S. Pat. No. 3,277,218.
However, only Ser. No. 932,244 discloses control over the size pellets produced by evaluating the finished pellet after it has been emitted from the rotary pelletizer. The other copending applications and patent disclose sensing a characteristic of the material within the pelletizer and projecting that parameter to the size of the finished pellets.
In this regard, pelletizer controls can be divided into two categories: those that control the quality of the finished pellet by evaluating some feature or characteristic within the pelletizer, and those that control the quality of the finished pellet by evaluating the pellet after it has been formed and ejected from the pelletizer.
Another copending application Ser. No. 974,456, filed Dec. 29, 1978, and assigned to the common assignee, discloses a control system for a pelletizer.